feat(http2): Make HTTP/2 support an optional feature

cc #2251

BREAKING CHANGE: This puts all HTTP/2 methods and support behind an
  `http2` cargo feature, which will not be enabled by default. To use
  HTTP/2, add `features = ["http2"]` to the hyper dependency in your
  `Cargo.toml`.

Cargo.toml

@@ -31,7 +31,7 @@ http = "0.2"
 http-body = "0.3.1"
 httpdate = "0.3"
 httparse = "1.0"
-h2 = { git = "https://github.com/hyperium/h2" }
+h2 = { git = "https://github.com/hyperium/h2", optional = true }
 itoa = "0.4.1"
 tracing = { version = "0.1", default-features = false, features = ["log", "std"] }
 pin-project = "1.0"
@@ -56,6 +56,7 @@ tokio = { version = "0.3", features = [
     "fs",
     "macros",
     "io-std",
+    "io-util",
     "rt",
     "rt-multi-thread", # so examples can use #[tokio::main]
     "sync",
@@ -73,7 +74,16 @@ pnet = "0.25.0"
 [features]
 default = [
     "runtime",
-    "stream"
+    "stream",
+
+    #"http1",
+    "http2",
+]
+full = [
+    #"http1",
+    "http2",
+    "stream",
+    "runtime",
 ]
 runtime = [
     "tcp",
@@ -86,6 +96,10 @@ tcp = [
     "tokio/time",
 ]
 
+# HTTP versions
+#http1 = []
+http2 = ["h2"]
+
 # `impl Stream` for things
 stream = []
 
@@ -94,10 +108,11 @@ nightly = []
 __internal_happy_eyeballs_tests = []
 
 [package.metadata.docs.rs]
-features = [
-    "runtime",
-    "stream",
-]
+features = ["full"]
+rustdoc-args = ["--cfg", "docsrs"]
+
+[package.metadata.playground]
+features = ["full"]
 
 [profile.release]
 codegen-units = 1

src/body/body.rs

@@ -14,6 +14,7 @@ use http_body::{Body as HttpBody, SizeHint};
 #[cfg(feature = "stream")]
 use crate::common::sync_wrapper::SyncWrapper;
 use crate::common::{task, watch, Future, Never, Pin, Poll};
+#[cfg(feature = "http2")]
 use crate::proto::h2::ping;
 use crate::proto::DecodedLength;
 use crate::upgrade::OnUpgrade;
@@ -39,6 +40,7 @@ enum Kind {
         want_tx: watch::Sender,
         rx: mpsc::Receiver<Result<Bytes, crate::Error>>,
     },
+    #[cfg(feature = "http2")]
     H2 {
         ping: ping::Recorder,
         content_length: DecodedLength,
@@ -186,6 +188,7 @@ impl Body {
         Body { kind, extra: None }
     }
 
+    #[cfg(feature = "http2")]
     pub(crate) fn h2(
         recv: h2::RecvStream,
         content_length: DecodedLength,
@@ -273,6 +276,7 @@ impl Body {
                     None => Poll::Ready(None),
                 }
             }
+            #[cfg(feature = "http2")]
             Kind::H2 {
                 ref ping,
                 recv: ref mut h2,
@@ -325,10 +329,11 @@ impl HttpBody for Body {
     }
 
     fn poll_trailers(
-        mut self: Pin<&mut Self>,
-        cx: &mut task::Context<'_>,
+        #[cfg_attr(not(feature = "http2"), allow(unused_mut))] mut self: Pin<&mut Self>,
+        #[cfg_attr(not(feature = "http2"), allow(unused))] cx: &mut task::Context<'_>,
     ) -> Poll<Result<Option<HeaderMap>, Self::Error>> {
         match self.kind {
+            #[cfg(feature = "http2")]
             Kind::H2 {
                 recv: ref mut h2,
                 ref ping,
@@ -348,27 +353,34 @@ impl HttpBody for Body {
         match self.kind {
             Kind::Once(ref val) => val.is_none(),
             Kind::Chan { content_length, .. } => content_length == DecodedLength::ZERO,
+            #[cfg(feature = "http2")]
             Kind::H2 { recv: ref h2, .. } => h2.is_end_stream(),
             #[cfg(feature = "stream")]
             Kind::Wrapped(..) => false,
         }
     }
 
     fn size_hint(&self) -> SizeHint {
-        match self.kind {
-            Kind::Once(Some(ref val)) => SizeHint::with_exact(val.len() as u64),
-            Kind::Once(None) => SizeHint::with_exact(0),
-            #[cfg(feature = "stream")]
-            Kind::Wrapped(..) => SizeHint::default(),
-            Kind::Chan { content_length, .. } | Kind::H2 { content_length, .. } => {
+        macro_rules! opt_len {
+            ($content_length:expr) => {{
                 let mut hint = SizeHint::default();
 
-                if let Some(content_length) = content_length.into_opt() {
+                if let Some(content_length) = $content_length.into_opt() {
                     hint.set_exact(content_length);
                 }
 
                 hint
-            }
+            }};
+        }
+
+        match self.kind {
+            Kind::Once(Some(ref val)) => SizeHint::with_exact(val.len() as u64),
+            Kind::Once(None) => SizeHint::with_exact(0),
+            #[cfg(feature = "stream")]
+            Kind::Wrapped(..) => SizeHint::default(),
+            Kind::Chan { content_length, .. } => opt_len!(content_length),
+            #[cfg(feature = "http2")]
+            Kind::H2 { content_length, .. } => opt_len!(content_length),
         }
     }
 }

src/cfg.rs

@@ -0,0 +1,9 @@
+macro_rules! cfg_http2 {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "http2")]
+            //#[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
+            $item
+        )*
+    }
+}